The present invention relates especially, but not exclusively, to the concentration of sunlight, and to the use of such concentration with advanced high-concentration photovoltaic cells. The present concentrators can also be used as collimators for a variety of light sources, especially light emitting diodes, including an array of light emitting diodes.
The highest efficiency in photovoltaic cells currently commercially available has been achieved by the triple-junction designs of the Spectro-Lab Corporation, but they are too expensive to use without optical concentration, except for the space-power applications for which they were originally developed. “Concentration” is the gathering of light rays so that the light falls at a higher intensity on a smaller area. Two-dimensional concentration for terrestrial solar power generation requires solar tracking, because concentrators rely on the incident light rays being substantially parallel and usually aligned in a known direction relative to the concentrator. Concentration is only effective on clear days, when direct-beam sunlight predominates. In cloudy weather there can be little or no concentration. Consequently, in climates with a high proportion of cloudy weather only silicon cells, less than half as efficient, are affordable. Direct sunlight may be treated as substantially uniform illumination from a circular source at infinity of angular diameter 0.530.
It is also desirable to be able to focus and concentrate radiation from an area just surrounding the visible sun disk, which is known as the circumsolar radiation. This radiation comes from a region surrounding the sun within an annular ring from 0.53° (the direct beam cone) to approximately 40 in diameter. The circumsolar radiation can be as much as 5 to 10% of the total energy from the sun. It is typically at its highest level when there is very light cloud cover over the sun (with high humidity) and at its lowest levels (on the order of 0.1%) when there are very clear sky conditions (with low humidity). In order to capture the circumsolar radiation a wide acceptance angle concentrator is needed.
The scale and magnitude planned for the Solar America Initiative of the United States Department of Energy highlights the great need for continual performance improvements and cost reductions in solar energy generators. While distributed household systems will be fixed flat panels, large-scale utility systems will most likely utilize tracking concentrators, because the high cost of the most efficient, triple-junction cells mandates their use at the highest geometric concentration they can withstand (about 1000 suns, given proper heat-sinking).
Such concentration also requires accurate two-axis tracking, but tracking systems become more expensive if an accuracy of one-degree maximum angular error is required, as is the case for conventional Fresnel lenses and parabolic mirrors. Such angular accuracy affects all the tolerances in the system: optical surface manufacturing accuracy and finish, precision of assembly, the stiffness of the supporting structure, and the mechanical accuracy of the tracker.